Thin metal layers are useful in many industrial applications. For example, the advanced copper interconnect technology used in semiconductors requires the use of thin metal layers to act as diffusion barriers to prevent Cu diffusion into the insulating dielectric or to reduce electromigration at the surface of Cu lines. Electroless plating is a process for plating material onto a surface in the absence of an external electric current. Plating reagents using electrochemical phenomena, e.g. oxidation/reduction reactions, have been used for electroless plating. Specifically, the deposition of a few tens of nanometers of a Co or Ni alloy additionally comprising a few percent of an additional component (typically W, P, B or Mo) by an electroless process has been considered. For example, the alloy Co(P) has been identified as useful by E. J. O'Sullivan et al., “Electrolessly Deposited Diffusion Barriers for Microelectronics,” IBM J. Res. Dev., 42 (5) September 1998, p. 607.
The details of the plating mechanisms in electroless plating processes seem to be poorly understood, and are the subject of many papers in the academic and trade literature. For a review of electroless plating, see Mallory and Hajdu, “Electroless Plating: Fundamentals and Applications,” American Electroplaters and Surface Finishers Society, Orlando. The use of two or more reducing agents is sometimes necessary, one to initiate plating on the substrate material, and one to continue plating onto the partially plated layer. The reducing agent(s) can also be used to supply desired additional components. The use of two reducing agents that are hypophosphite and a dimethylamine borane complex is discussed in Mital et al., “Studies of Elecectroless Nickel Deposition Using Electroanalytical Techniques,” Metal Finishing Mag., June 1987, p. 87. The use of hypophosphite and a dimethylamine borane complex reducing agents to plate nickel onto copper is discussed in Watanabe et al., “Electroless Nickel Plating Using DMAB as 2nd Reducing Agent,” J. Jap. Inst. For Interconnecting and Packaging Electronic Circuit, 12 (4) 1997, p. 231. Electroless plating is also discussed in US Patent Application Nos. 2003/0113576 and 2004/0026786 and U.S. Pat. Nos. 5,169,680; 5,695,810; 5,824,599; and 6,696,758.